With his physical play and goal scoring ability, Shanahan scored 656 goals in his NHL career spanning over 1,500 NHL games and, at the time of his retirement, was the leader among active NHL players for goals scored. Shanahan is the only player in NHL history with over 600 goals and 2,000 penalty minutes.

Shanahan was drafted by the New Jersey Devils second overall in the 1987 NHL Entry Draft after Pierre Turgeon. Expectations for Shanahan were high after a stellar career with the London Knights of the Ontario Hockey League (OHL), with whom his number 19 has been retired. In his rookie season with the Devils, in 1987–88, he scored 26 points in 65 games as an 18-year-old. The following season, in 1988–89, he improved to 22 goals and 50 points. In his third NHL season, he emerged as a point-per-game producer with 72 points in 73 games and a top scorer with the Devils; his 30 goals finished tied for second in team goal-scoring behind John MacLean.[3] In his fourth and final year of his initial stint with the Devils in 1990–91, he scored 29 goals and 66 points. At the age of 22, Shanahan was already an established scorer in the NHL. He had also played well in the Devils' playoff runs.

Becoming a free agent following the 1990–91 season, Shanahan was signed by the St. Louis Blues on July 25, 1991.[4] According to the collective bargaining agreement, he was a restricted free-agent, and therefore, the Devils were due compensation. Ordinarily, this compensation would be in the form of draft picks, but the Blues already owed four first-round draft picks to the Washington Capitals for signing defenceman Scott Stevens the previous year. The Blues made an offer for compensation that consisted of Curtis Joseph, Rod Brind'Amour and two draft picks even further down the road.[citation needed] However, the Devils were only interested in Scott Stevens. An arbitrator eventually decided that Stevens was to be the compensation, so Shanahan joined the Blues in exchange for Scott Stevens.[4]

While Shanahan's first season for the Blues yielded similar statistics to his seasons with the Devils, he would reach another level in 1992–93 with 51 goals and 94 points in 71 games. He finished second in team goal-scoring to Brett Hull and third in team point-scoring overall. Continuing at that pace the next season, in 1993–94, he recorded personal bests of 52 goals, 50 assists and 102 points. In addition to leading the Blues in points, he was named to the 1994 NHL All-Star Game at mid-season and the NHL First All-Star Team at the end of the year.

During the 1994–95 NHL lockout, Shanahan played 3 games for Düsseldorf EG of the Deutsche Eishockey Liga, managing to score 5 goals and tally 3 assists in his short stay overseas. When the NHL started back up, he continued to play well for the Blues, recording 41 points in the lockout-shortened season. In the playoffs, he led the team in scoring with 9 points in 5 games.

Shanahan finished off the season with his usual productivity, scoring a total of 47 goals for the season, and was named to the 1997 NHL All-Star Game. In the playoffs, Shanahan contributed 9 goals and 8 assists, helping the Red Wings to their first Stanley Cup since 1955. They repeated as Cup champions the next year, despite an off-season for Shanahan, in which he managed just 57 points. The following season, in 1998–99 NHL season, Shanahan continued at that pace with 58 points, but was still invited to another All-Star Game. Going into the playoffs as back-to-back Stanley Cup champions, the Red Wings were eliminated by the rival Colorado Avalanche. In 1999–2000, Shanahan scored 41 goals, indicating a return to his usual form. After the season, he was named to the First All-Star Team for the second time in his career. He put up 76 points in the 2000–01 NHL season, although Detroit was upset in the first round of the playoffs to the Los Angeles Kings.

The 2001–02 season was a banner one for both Shanahan and the Red Wings. Having picked up future Hall-of-Famers Brett Hull, Luc Robitaille and Dominik Hašek in the off-season, the team was primed to win its third Cup since 1997. They cruised to victory and Shanahan continued to play a big role in their success, scoring 37 goals during the regular season and 19 points in their victorious Stanley Cup run. Shanahan also picked up an Olympic gold medal in Salt Lake City with Team Canada and was named to the Second NHL All-Star Team. The season was also of particular statistical significance for Shanahan, as shortly preceding his Olympic gold medal victory, he recorded his 1000th point in the NHL with two goals against Marty Turco in a 4–2 victory over the Dallas Stars on January 12, 2002.[5] Then, late in the season, Shanahan also reached the 500-goal mark, scoring the game-winner against Patrick Roy in a 2–0 victory over the Colorado Avalanche on March 23, 2002. The win also marked a team accomplishment as it clinched a President's Trophy as the top ranked regular-season team.[6]

In the season following their third Stanley Cup, Shanahan scored 30 goals and 68 points and won the King Clancy Memorial Trophy at the end of the year for his humanitarian efforts. In the following season, however, his production dipped to 25 goals and 53 points, his lowest totals in fifteen years. After a one-year absence due to the 2004–05 NHL lockout, Shanahan showed yet another return to form in the 2005–06 NHL season, tallying an impressive 40 goals and 81 points, good for third among Red Wings in scoring.

Shanahan (with the "A" on his jersey) warms up during a September 2007 pre-season game.

Shanahan became a free agent following the 2005–06 season and signed a one-year, $4 million deal with the New York Rangers.[7] After completing a successful nine-year stay in Detroit, he expressed a desire to move on in his NHL career, stating "It really came down to an instinct I had. Detroit has a great past and a great future ahead of them as well, but I guess I just felt that maybe I was identified with the past a little bit more than the future."

Shanahan began his Rangers' career by scoring his 599th and 600th career goals against Olaf Kölzig on October 5, 2006, in a 5–3 season opening win against the Washington Capitals at Madison Square Garden.[8] With assists coming from Petr Průcha on both goals, he became just the 15th player in NHL history to reach the 600-goal mark.[citation needed] Shortly thereafter, on November 14, 2006, Shanahan received the inaugural Mark Messier Leadership Award, an award given monthly to a player selected by Mark Messier who best exemplifies leadership skills on and off the ice. Then, selected to his eighth All-Star Game, he was named captain of the Eastern Conference for the 2007 All-Star Game.[9] On February 1, 2007, he made headlines after expressing frustration in a press conference about his perception that NHL referees are biased against team captain Jaromír Jágr.[10] Later in the month, he was involved in a severe on-ice collision with Philadelphia Flyers forward and former Red Wings teammate Mike Knuble in a game on February 17. Shanahan and Knuble caught each other skating in opposite directions as Shanahan was headed for the bench, at which point Shanahan hit his head on the ice and was left unconscious for ten minutes. He was carried off on a stretcher and taken to hospital where he was released the next day.[11] After missing 15 games, Shanahan returned to the lineup in time for the playoffs where the Rangers were defeated by the Buffalo Sabres in the second round. Shanahan completed his first season with the Rangers fourth in team scoring with 62 points in 67 games as an alternate captain to Jagr.[8]

After re-signing to another one-year contract with the Rangers,[12] Shanahan struggled to produce offensively as his points total dipped to just 46 points in 2007–08, his lowest total since his rookie season in 1987–88. With his contract expiring in the off-season, he was not tended an offer by the Rangers, believed to be a result of the Rangers' pursuit of free agent Mats Sundin.[4]

Unable to come to terms with the Rangers, Shanahan sat out the first half of the 2008–09 season. Then, on January 10, 2009, it was announced that Shanahan agreed to join the New Jersey Devils for his second stint with the team. Four days later, on January 14, the terms of the contract were finalized and Shanahan signed a one-year, $800,000 pro-rated contract.[13] The time between Shanahan's departure from and return to the Devils was 17 years, 294 days, the longest gap in tenure with one team in NHL history.[14] Playing in his first game back with the Devils since the 1990–91 season, he scored the first goal of the game against the Nashville Predators on a 5 on 3 power play by toe dragging the puck around the opposition player and then shooting it on the pad side on January 19 in a 3–1 win.[15] On August 5, 2009, Shanahan agreed to a one-year deal with the Devils returning for a 22nd season,[16] to play during the 2009–10 season. This would have been Shanahan's 6th season as a New Jersey Devil. However, on October 1, 2009, the Devils and Shanahan parted ways, with Brendan Shanahan saying "When I signed this past summer, Lou Lamoriello, Jacques Lemaire, and I agreed that if we were unable to find a suitable fit in which I would be able to compete and contribute at the level I expect from myself, then I would simply step aside." Shanahan had played just four preseason games of the 2009–10 season. He scored the Devils last preseason goal that year, on one of his last NHL shifts.[17]

During the 2004–05 NHL lockout, Shanahan was the mastermind of what was dubbed "The Shanahan Summit," a two-day conference in Toronto. It gathered players, coaches, and other influential voices to discuss improvements to the flow and tempo of the game. Ten recommendations were presented to the League and Players Association.

According to The Hockey News, Shanahan holds an unofficial NHL record for most modern day career Gordie Howe hat tricks with 17.[18] However, not all teams have kept records of this feat, and it is even believed that Gordie Howe himself only officially had two. According to a Yahoo! Sports article, Shanahan would choose to go into the Hall of Fame as a Red Wing, if he had to choose.[19]

On November 17, 2009, Shanahan officially announced his retirement after 21 years in the NHL. Shanahan said, "I would like to thank my family and all of the friends who have helped me achieve and maintain my childhood dream of playing in the National Hockey League", Shanahan said in a news release. "I am enormously grateful to all of my coaches and teammates I've had the privilege of learning from and playing along side of, throughout my career. While I always dreamed of playing in the NHL, I can't honestly say that I would have ever imagined that I'd be this fortunate and blessed. I would like to sincerely thank everyone who has helped me fulfill this dream."[20]

In December 2009, Shanahan accepted an offer from the NHL to become the NHL's Vice President of Hockey and Business Development.[21] "In a broad sense, I think obviously, I am going to be another voice in the hockey ops, but at the same time people like John Collins and Gary and Bill are going to allow me and teach me the business of hockey," Shanahan told NHL.com. "What I was excited about in their offer to bring me on board is that it was wide open for me. There was not going to be any room with a closed door and I would be given an opportunity to see and learn. As time goes by there will be some days where my role is more hockey specific and some days where my role is more business or marketing specific."

On June 1, 2011, Shanahan succeeded Colin Campbell as the NHL's chief player disciplinarian. In his very first preseason as director of player discipline, Shanahan delivered multiple suspensions to players for illegal hits. Each ruling is accompanied by a video in which Shanahan offers the official view of the offending play, and how it did or did not constitute a breach of NHL regulations. The media has since coined the term "shanaban".

On April 11, 2014, Shanahan was officially announced as the Toronto Maple Leafs president and alternate governor. Shanahan is expected to oversee all operations for the hockey club. On the same day, the NHL announced that Stéphane Quintal would replace Shanahan as the league's chief disciplinarian.[22]

Shanahan married his wife Catherine (who is the ex-wife of his former teammate Craig Janney) on July 4, 1998. They have three children.[24][25] Shanahan became a United States citizen on May 17, 2002. He lives in New York City during the on-season.[26]